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Magnetic pickup aperture
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10102&t=13937		 Page 1 of 1
Author:  erikbojerik [ Mon Oct 08, 2007 5:02 am ]
Post subject: 
I'm engaged in a debate about the 3D sensing aperture of standard
magnetic pickups, specifically the part of the coil that actually
generates most of the current.

 

One side maintains that most of the current is generated at the top of
the coil, closest to the strings.  The other side maintains that
all parts of the coil generate current equally.



The example being debated is a single-bobbin stacked humbucker, with
one coil on top wound CW, and another coil on the bottom wound CCW...at
least this is how I understand how such a thing is created.



So...which is it?  I won't say which side of the debate I'm on...except to say its not the second one. 



(Rick, if you're home I'd love your input on this...anyone else too of course)
Author:  Rick Turner [ Mon Oct 08, 2007 6:07 pm ]
Post subject: 
First of all, I consider aperture to be the length of the string sensed, not the depth of the coil.   Secondly, magnetic phenomena drops off as the square of the distance.   To think that the bottom turns of wire...the ones farthest away from the strings contribute as much to the voltage as the closest ones is simply to not understand how any of this works.   There are things you can do to make the lower turns more efficient, and that's one of the "secrets" of string surrounding pickups like the Rickenbacher horseshoe pickups or some of the National/Valco lap steel pickups.

To test this, wind about 1000 turns of wire on a Strat or Tele style bobbin with the two bobbin plates spaced about 3/16" apart.   Use all the same length magnets.   Put one bobbin at the very end of the magnets, and the other bobbin at that 3/16" distance.   Now try the pickup with the coil closest to the strings.   Now flip it upside down so the magnets are at the same distance, but the coil is down at the bottom.   Tell me which is louder... No, don't bother; I already know. So now show that to someone who thinks that distance doesn't matter...

A magnetic pickup works in a 3D matrix of flux, coil positioning, and string motion. Everything counts.   

"Pancake" coils are inherently more efficient...but such a pickup will sound different.   As I said, everything counts...
Author:  erikbojerik [ Mon Oct 08, 2007 10:01 pm ]
Post subject: 
Thanks Rick, that confirms what I suspected.



Interestingly, one of the guys in this debate did exactly the test you
describe, except instead of actually putting the picking in a guitar,
he measured the output in a test rig he set up for measuring frequency
response.  As I understand it, he's running a sweep through a
headphone driver to alter the field around the pickup.  He claims
the "output" of the pickup is the same in either orientation (within
2%...).  I personally think he's perturbing the field over too
large a 3D volume to do correctly the kind of test you suggested. 
I told him to try it in an actual instrument.



My understanding of how this works goes something like this (please
feel free to slap me down where I'm wrong), using the single coil as an
example.  The magnetic field of the pickup is set up by the
magnetized pole pieces, and is disturbed by the strings only because
they are ferromagnetic and so have attained a localized (along their
length) induced magnetism from the nearby pole pieces.  Their
vibration peturbs the magnetic field lines, and where the perturbed
field lines cross the coil, that is where current is actually generated
(moved).



Because the perturbation due to the strings the field falls off as
1/d^2, to me it would make perfect sense that the coil would generate
more current closer to the strings (tops of the pole pieces) than it
would further away (flipped over).



Have I got it?
Author:  WaddyThomson [ Tue Oct 09, 2007 12:42 am ]
Post subject: 
"perturbation" 

I'm sorry this is a family forum.  Words like this are not allowed, cause they really confuse me!

Author:  Bob Garrish [ Tue Oct 09, 2007 1:07 am ]
Post subject: 
[QUOTE=WaddyT] "perturbation" I'm sorry this is a family forum. Words like this are not allowed, cause they really confuse me!
[/QUOTE]

Nice, +10 wit points for you
Author:  erikbojerik [ Tue Oct 09, 2007 1:31 am ]
Post subject: 

[QUOTE=Bob Garrish] [QUOTE=WaddyT] "perturbation" I'm sorry this is a family forum. Words like this are not allowed, cause they really confuse me!

[/QUOTE]



Nice, +10 wit points for you [/QUOTE]



Hey, at least it's better than "disturbation"...

At least pickups won't make you go blind...
Author:  WaddyThomson [ Tue Oct 09, 2007 5:40 am ]
Post subject: 

[QUOTE=erikbojerik]
[QUOTE=Bob Garrish] [QUOTE=WaddyT] "perturbation" I'm sorry this is a family forum. Words like this are not allowed, cause they really confuse me!

[/QUOTE]



Nice, +10 wit points for you [/QUOTE]



Hey, at least it's better than "disturbation"...

At least pickups won't make you go blind...

[/QUOTE]

Who Said That?
Author:  Rick Turner [ Tue Oct 09, 2007 4:03 pm ]
Post subject: 
Note, you will find more induced current in more distant turns with an "iron load" coil core as opposed to a coil that has a ceramic magnet core.   String motion will vary the flux at the bottom of a pole magnet...a bit...but won't do that with a ceramic magnet.   

Like I said, everything changes everything with mag pickups.
Author:  erikbojerik [ Tue Oct 09, 2007 11:11 pm ]
Post subject: 

Getting back to 2D aperture...I have heard it said that narrow coils (like
Strat pickups) have a smaller aperture than a wider coil with identical
magnets (like an R90) with identical DC resistance.  I know they
will sound different and have different inductance due to the coil
shape, baseplates, etc etc.



But with identical magnets as pole pieces, wouldn't the actual magnetic aperture be the same?
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